A satellite communication system often includes a satellite, a central earth station or a hub, and a plurality of terminals. The hub transmits data to the terminals over a forward link (FL) carrier signal and each terminal receives the FL signal and extracts the data destined to it. The terminals transmit data to the hub, usually over several return link (RL) channels, and the hub uses receivers for receiving the transmissions of the terminals.
The RL resources are often managed using one of two methods. In the first method, known as Single Channel per Carrier (SCPC), each terminal is assigned a channel on which it can continuously transmit (i.e. without interrupting the transmitted carrier). Continuous transmission allows use of very efficient waveforms. In the SCPC method, since the number of channels is the same as the number of terminals, the hub needs to use a receiver per terminal. In the second method, based either on Time Division Multiple Access (TDMA) or on Multi-Frequency TDMA (MF-TDMA), each terminal is assigned time intervals (timeslots) on a channel or on several channels (at different times), and the terminal transmits in bursts over the assigned timeslots. In the TDMA-based methods, since several terminals share a channel and the number of receivers the hub needs to use is in accordance with the number of channels, the hub needs fewer receivers than the number of terminals. On the other hand, since the transmissions are burst transmissions, the efficient waveforms used in the SCPC method are not usable with TDMA based methods.
The spectrum available for satellite communication is limited (e.g. about 750 MHz in Ku-band and about 2500 MHz in Ka-band, per polarization). To increase the capacity of satellite communication systems, the spectrum is reused through division of the service area into multiple beams or spot beams and employing a frequency reuse scheme. However, multiple hubs are also needed in order to support the tens, hundreds, and even thousands of spot beams. Using the conventional techniques (e.g. as described above) would result in requiring a very large number of hubs comprising huge amounts of equipment, thus dramatically increasing the cost of satellite communication systems.
One method for dividing or sharing the available spectrum between user beams is known as Beam Hopping. The beam hopping method is supported by the “Digital Video Broadcasting (DVB); Second generation framing structure, channel coding and modulation systems for Broadcasting, Interactive Services, News Gathering, and other broadband satellite applications; Part 2: DVB-S2 Extensions (DVB-S2X)”, also known in short as the DVB-S2X standard (ETSI EN 302 307-2). In the beam hopping method, the satellite switches (practically instantaneously) the hub's feeder beam, which includes the FL carrier (and the associated RL channels), between several user beams (i.e., that service terminals), e.g., in a TDMA manner. This switching allows the hub to support a larger service area (i.e. more spot beams) using a given amount of equipment and dynamically adjust the capacity allotted to each user beam (e.g. by adjusting the switching intervals). Consequently, the FL carrier is present at each beam for only a fraction of the time (and the terminals may transmit to the hub only during that fraction of the time), making it necessary for the hub (and the terminals) to synchronize data transmission with the beam hopping pattern (i.e. transmit the right data at the right time). Perhaps equally important, to support beam hopping it is necessary for each terminal to maintain its FL receiver synchronized even when the FL carrier signal is not present, or quickly relock on the FL carrier signal when the FL carrier signal becomes present, to enable reception of the FL carrier signal without losing any data being transmitted. In a way, the beam hopping method requires the terminals to employ a burst receiver capable of receiving a waveform designed for continuous transmissions, e.g. such as the DVB-S2X waveform.